The present invention generally relates to wireless communications by the multi-input multi-output (MIMO) and the multicarrier method, and, more particularly, to a channel estimation device, a channel estimation method, and a wireless receiver that are used in such wireless communications.
In this field of technology, studies are being made so as to realize high-capacity high-speed information communication systems of the present and next generations and later generations. Attention is being drawn to the MIMO method to increase the communication capacity, and to the multicarrier method to reduce the interference in a multipath propagation environment. Particularly, more attention is being drawn to the orthogonal frequency division multiplexing (OFDM) method.
FIG. 1 is a schematic view of a MIMO communication system that includes a transmitter 102 and a receiver 104. As shown in FIG. 1, in accordance with the MIMO method, different signals are simultaneously transmitted from transmission antennas 106-1 through 106-N at the same frequency. These transmission signals are received by reception antennas 108-1 through 108-N. For ease of explanation, the number of transmission antennas and the number of reception antennas are both N in this example. However, the number of transmission antennas may be different from the number of reception antennas.
FIG. 2 is a schematic view of the receiver 104. Reception signals y1 through yN received by the respective reception antennas 108-1 through 108-N are input to a signal detector 202. The reception signals y1 through yN are also input to a channel estimation unit 204. Based on reception signals including known pilot signals at both sides of transmission and reception, the channel estimation unit 204 determines a channel impulse response (CIR) value or a channel estimate value, thereby performing channel estimation.
FIG. 3 is a schematic view illustrating the structure and operation of the channel estimation unit 204. In this example illustrated in FIG. 3, pilot signals are time-multiplexed in one frame. Here, the pilot signals are represented as the signal sequence of “+1, +1, −1, −1”, and these signals are contained at four locations in the frame, In the frame, the portions other than the pilot signals are equivalent to data signals or the like. As indicated by the broken line in FIG. 3, at multipliers 301 through 304 in the channel estimation unit 204, the pilot signals extracted from the frame are multiplied by the signals held by the channel estimation unit 204. The multiplication results of the respective multipliers 301 through 304 are added up at an adder 305. The output after the adding operation represents the channel estimate value.
Referring back to FIG. 2, the signal detector 202 separates the signals transmitted from the respective transmission antennas 106-1 through 106-N, based on the reception signals supplied from the reception antennas 108-1 through 108-N and the channel estimate value supplied from the channel estimation unit 204. The separated signals are then supplied to a channel decoder to perform further decoding operations.
An example of the conventional channel estimation is disclosed in Japanese Laid-Open Patent Application No. 2003-338779, for example.
By the above described MIMO method, channels (propagation paths) are presumably set between the transmission antennas and the respective reception antennas, and generally have different channel variations. Especially, in the case where the multicarrier method is employed, a channel estimate value needs to be set for each sub carrier. Accordingly, in the case where the MIMO method or the multicarrier method is employed, it is necessary to accurately determine a very large number of channel estimate values. Otherwise, appropriate signal separation will become difficult at the signal detector 202. For example, the number of transmission antennas and the number of reception antennas are both 2, the QPSK modulation method is employed, and eight sub carriers are used. In such a case, there are four types of possible signal points for each one transmission antenna. Therefore, it is necessary to examine 16 combinations of signal points for each of the eight sub carriers at the reception side.
In the conventional channel estimation illustrated in FIGS. 2 and 3, however, the precision in the channel estimation is not necessarily high, and signal separation might not be appropriately performed in some communication environments. As the number of antennas, the number of sub carriers, and the number of signal points in the multivalue modulation increase, this problem becomes more severe.